HarryD

Cross posted at the Bird...
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A few weeks ago, I went to start my CIS car and there was a loud boom followed by my being unable to start the car. Knowing I had a flapper in the air box, I was sure I did not blow up my air box but something was not right.

I opened the engine room and saw that one of my 30+ year old rubber straps holding the air cleaner cover to the box had broken and was askew. I removed the cover and saw the flapper valve lying next to the hole it formerly occupied.

I pushed the valve back into the hole and used some 3M Blue Tape to hold the air cleaner in place and started the car. It started without a hitch.

When I got home, I cleaned up the old epoxy, cleaned the area with carb cleaner, and used JB Weld to glue the valve back into place. I ordered two replacement straps and installed them as well.

After waiting 2 days, I started the car for the first time. I noticed it started easier than it has ever and the idle was smoother.

I think the improvements came from the fact that, over time, the valve had developed a slight crack at the glue joint that allowed a small bypass of air.

Anyone who has one of these things should periodically check for failure of the glue and reglue as needed to ensure a leak free install.

Daniel Dudley

Do you have leaky plug wires ? A crossfire could cause another backfire.

Maybe someone else could come up with another explanation, but this is a likely culprit.

Daniel Dudley

Plug wires are a common source of backfire. I'm not a CIS genius, but can someone tell us if the mixture could cause this ? I have never read anything about the source of CIS backfires.

theiceman

I think Harry had it nailed . I am always argueing my defence of no pop off valve with people. I In fact i was back at it again last Saturday. I think ( in this case anyway ) it is like fulfilling ones own prophesy. The leaking by the pop off valve caused the mixture to be way to lean which is critical on starting. The lean condition caused the backfire which then required a pop off valve. The question I have is this. If you had never had a pop off valve would you have had these issues.

BTW thanks for sharing .... I have a feeling this will be along thread as we just seam to love talking about this issue.

incidentally my car is a 78 , never had a pop of valve for 30 years, when i did blow up the air box ( my fault, read my sig ) i replaced it and did NOT install a pop off.

HarryD

Ice:

I see your point but the Pop off valve makes me feel better. But like many things, as you add complexity, you add additional failure modes. If you have one, it requires some level of maintenance, just like every other part of the car.

race911

I had so many of those things fall out when I had my shop in the '80's-early '90's from owner installs. Owners loved it when the towing charge was more than my repair bill!

HarryD

Interesting. What is your recommendation for a permanent repair?

scottrx7tt

i just bought my car, it has almost 165k on it, and no pop off valve, if it has not blown up by now, it probably wont.

theiceman

and it also tells you your car is in very good tune as no pop off is necesary. Sounds like you got a good one..

Peter Zimmermann

You did it! JB Weld is the best, a "permanent" repair isn't possible...

You are so wrong!!! However, before commenting further what year is your car?

Equally important as the mixture is the cold control pressure!

HarryD

Pete,

Thanks. Good to know I used an appropriate material.

Also, knowing how you feel about Pop off valves, the rest of your comments are interesting.

scottrx7tt

You are so wrong!!! However, before commenting further what year is your car?



Equally important as the mixture is the cold control pressure![/QUOTE]

my car is an 82

theiceman

i think you have the difuser in the airbox.

Peter Zimmermann

my car is an 82[/QUOTE]


*This is not a trick question! In the engine compartment, the airbox (black plastic) has a flat side directly below the fuel distributor that faces to the right of the car. On that flat surface does your car have a rectangular blue sticker (about 1" x 1.5"), or glue residue from a sticker?

(ice: Correct, the original airbox on all '82 models came with the internal diffuser).

BlackPearl

FWIW I have a pop-off valve and it's saved me from replacing the air box. JB weld or similar heat-tolerant epoxy was recommended when my valve "popped" out. I used JB Weld.

the cause of my "pop" may have been (is) the fact that I've tuned my car to be ever-so-slightly rich for track use. When it sits in the garage waiting for that next country-road drive or track event, it starts a little more cantankerously.

One important find: I use a K&N filter that doesn't make much room for the pop-off valve. I had to "modify" the K&N to allow a "clean opening" for the pop-off valve to work properly.

Personally, I like the valve. Obviously, others may disagree.